WASHINGTON  U.S. residents can breathe a bit easier than they did a decade ago, as the number of days that air quality was deemed unhealthy has fallen, according to a report by the American Lung Association Thursday.

The report found real improvement in air quality over much of the United States, due in part to reduced emissions from power plants.

Significant benefits were seen in the eastern United States, said Janice Nolen, the association's director of national policy.

"It's encouraging news, we're very pleased at the improvement," Nolen said by telephone. "However, cleaner is not clean enough."

While power plants are emitting less foul air, the report said pollution from marine sources -- tug boats, ferries, recreational vessels -- and diesel-powered trains are still major polluters in port cities like Houston, Los Angeles, New York, Chicago and Pittsburgh.

To gauge improvement, researchers looked at the number of days deemed to have unhealthy air from 2002 to 2004, based on ozone monitors in 735 counties around the country, and found there were under 8,500 days where the air was unhealthy to breathe.

That compares favorably with the numbers from 1996 to 1998, when ozone monitors in 678 counties found more than 10,200 unhealthy air quality days, said Nolen.

Even so, over half the U.S. population -- over 150 million people -- live in the 369 counties that have unhealthy levels of smog or soot in the air, the report found.

This can pose risks to people with asthma, the elderly and those under the age of 18, those with chronic bronchitis and emphysema, cardiovascular disease and diabetes.

Nearly 15 percent of the U.S. population live in 34 counties with unhealthy levels of both smog and soot, known to scientists more specifically as ozone and particle pollution.

Cities that rank among the worst in the nation for both pollutants include: Los Angeles, Bakersfield, Fresno, Hanford and Visalia in California; New York City; Newark, New Jersey; Bridgeport, Connecticut; Washington D.C.; Baltimore; Pittsburgh, Pennsylvania; Cleveland, Ohio, and St. Louis, Missouri.